Liquid Fuels as Diesel Engine Fuel Mixture or Light Heating Oil

ABSTRACT

Liquid fuels such as a diesel engine fuel mixture or a light heating oil have as a main component, a fossil diesel engine fuel and/or a biodiesel and/or a vegetable oil and additives. The object is to provide an alternative mixture component for diesel engine fuels or light heating oil which is suitable not only for fossil diesel engine fuels but also biodiesel or vegetable oil, which may be produced inexpensively and which meets the requirements as a pure bio propellant. It is proposed that diesel engine fuel mixture or light heating oil contains, as a mixture component, diethyl oxalate. The heating oil can be made of a mixture which originates from the refining of crude oil and is admixed with up to 25% by volume diethyl oxalate. In addition, for the heating oil, use can be made of a mixture which consists of vegetable oils and up to 50% by volume diethyl oxalate.

The invention relates to liquid fuels as diesel engine fuel mixture or light heating oil, wherein the diesel engine fuel mixture, as main component, contains fossil diesel engine fuel and/or biodiesel and/or vegetable oil and additives which are conventional per se.

The main sources of diesel engine fuel (also known as diesel or diesel oil) and light heating oil are the so-called middle distillates which are extracted in the refining of crude oil in the “medium” boiling range (180° C. to 360° C.).

The main components of this mineral or fossil diesel engine fuel are alkanes, cycloalkanes and aromatic hydrocarbons with about 10 to 22 carbon atoms per molecule and a boiling point between 170° C. and 390° C. The sulphur content of diesel engine fuels should be as low as possible to prevent an increase in soot build-up and emission values.

The components and properties of fossil diesel engine fuel are regulated by DIN EN 590. This standard also restricts the sulphur content, which is to be gradually reduced to 10 mg/kg by 2009, a target which has already been achieved in some countries.

In recent years, the fuel known as biodiesel has also achieved a certain degree of importance as a fuel. Biodiesel is a mixture of methyl esters of the fatty aids of various vegetable oils and fats. Its properties are prescribed in DIN EN 14214. Its advantages are that it can be admixed with fossil diesel in any ratio and that it also meets the requirements of diesel fuel relating to flammability. Biodiesel has a higher viscosity than fossil diesel engine fuel which can lead to problems with highly advanced engines. However, the advantages of biodiesel lie in its high lubricating action for the fuel injection pumps, its high ignition quality and the comparatively low amount of soot emissions produced on combustion. However, the low volatility of biodiesel presents a problem because it can lead to an accumulation of fuel in the engine oil, which in turn means that the oil must be changed more frequently because of the low thermal stability of the biodiesel and the high stresses on the engine oils.

Small quantities of additives are also added to diesel engine fuels, such as ignition accelerators and additives for improving flow behaviour in cold conditions. In order to reduce emissions such as soot, nitrogen oxides and other substances from diesel engine fuels, diesel engines are also operated with a fuel mixture to which water has been added. Fuel mixtures are described in DE-PS 199 34 689 A1 which contain mineral diesel oil, water and emulsifiers as well as other additives where required. The proportion of water contained in the fuel mixture does not contribute to energy generation.

Additives are also known which contribute to combustion energy. For example, a mixture of fossil diesel engine fuel and vegetable oil is known as a diesel engine fuel mixture (DE 29 30 220 A).

However, vegetable oils lead to increased soot build-up and deposits in the engine oil because of their very poor volatility. A fuel composition is described in DE 10 2004 011 821 A1 which is composed of diesel oil, dry ethanol and an additive. Owing to the immiscibility of their components, the emulsions lack the storage stability required for fuels, and this is likely to lead to problems, as is generally known.

Ethers derived from glycerol are proposed as additives to diesel engine fuels in EP 0 641 854 B1. However, it is generally known that the esterification reaction is associated with a very time-consuming and costly synthesis. In addition, it is essential to prevent glycerol from getting into the fuel because of its immiscibility. Where this does occur, the ethers have to be cleaned in a series of complicated and time-consuming steps.

In accordance with the binding EU directive, diesel engine fuels must contain at least 2% bio propellant. This amount is due to increase to 5.75% by 2010. Hitherto, the only available source of this fuel was fatty acid methyl esters. Approximately 100 kg of free glycerol per tonne of biodiesel are produced during the manufacture of this substance. There is a limited market for glycerol which is already covered by existing production. In addition, fatty acid methyl esters are not in fact pure bio propellants since the methanol used for the transesterification of the fats/oils is of fossil origin and is not a bio product.

In the case of heating oils it is known only that certain additives can be added to these oils to improve their properties, such as combustion improvers to counteract soot build-up, odorants, flow improvers, viscosity-adjusting substances, pour-point depressing substances, as well as substances to improve their storage life. Experience has shown that vegetable oils, which have since become inexpensive, cannot be adapted with these additives to the burners for light heating oil of which there are very large numbers. Even when a flame can be generated by the momentary feeding in of light heating oil, it burns with a large build-up of soot and is spontaneously extinguished.

The aim of the invention is to provide an alternative mixture component for diesel engine fuels or light heating oil which is suitable both for fossil diesel engine fuels and for biodiesel or vegetable oil, is inexpensive to manufacture and meets the requirements as a pure bio propellant. In addition, a new diesel engine fuel is to be provided which is a pure bio propellant.

The aim is achieved in accordance with the invention by the features specified in claim 1. Advantageous embodiments are the subject of claims 2 to 6.

Claims 6 and 7 refer to the use of diethyl oxalate as a diesel engine fuel or as a mixture component for light heating oil. The otherwise usual additives can be added to this fuel or oil.

A mixture can be used as a heating oil which is derived from the refining of crude oil and is admixed with up to 25% by volume of diethyl oxalate.

In addition, a mixture can be used as a heating oil whose main component comprises vegetable oils and up to 50% by volume of diethyl oxalate.

The increase in the price of crude oil makes this an economically attractive alternative. The use of vegetable oils as a heating oil is made possible only by the addition of diethyl oxalate. Such a heating oil mixture has a low soot build-up and should contain a minimum of 20% by volume of diethyl oxalate.

With a diesel engine fuel mixture whose main component is a fossil diesel engine fuel containing diethyl ester of oxalic acid (diethyl oxalate) as a mixture component, the mixture component can be added in input amounts of up to 25% by volume. Higher inputs have an adverse effect on the lubricating properties of the diesel engine fuel mixture. This problem does not exist with biodiesel as the main component. However, the proportion of the mixture component should preferably not exceed 50% by volume. Diethyl oxalate can of course also be added to the mixtures of fossil diesel and biodiesel which are already known. Furthermore, diethyl oxalate can be added to unmodified vegetable oils in any ratio. This significantly reduces the viscosity of the vegetable oil or the fuel mixture, removing the need for a modified fuel supply. The production of a fuel mixture from diesel engine fuel (biodiesel and/or fossil diesel), vegetable oil and diethyl oxalate is also possible. The term, vegetable oil, also includes mixtures of different vegetable oils. Unlike fossil diesel engine fuel, biodiesel and vegetable oils can be admixed with diethyl oxalate in any ratio.

Diethyl oxalate can be manufactured inexpensively by the esterification of oxalic acid with ethanol. Even catalysts can be dispensed with in this process because of the relatively high dissociation constant of the oxalic acid. The reaction water released during esterification can simply be distilled off, e.g. with an entrainer, and the ester cleaned by simple water washing.

Ethanol and oxalic acid are extracted from carbohydrates. For example, ethanol is produced in large quantities by very economical microbial processes based on sugar or starch. Apart from the microbial processes, oxalic acid is manufactured on a large scale by the nitric acid oxidation of sugar or starch. In this process the nitric acid is reduced to nitrogen oxides which can in turn be turned back to nitric acid by spontaneous air oxidation and water.

Diethyl oxalate is very similar to conventional diesel engine fuel in its material properties, particularly as a fuel component for diesel engines. Its advantage lies in its flashpoint of 75° C., which is significantly higher than the minimum value of >55° C. required for mineral diesel engine fuel. Its very low freezing point (−40.6° C.) also contributes to improving the low temperature performance of other diesel engine fuels. Furthermore, diethyl oxalate is a pure bioproduct since all the carbon and hydrogen atoms are of biological origin.

Surprisingly, it was found that diethyl oxalate has additional advantages over biodiesel. When diethyl oxalate is subjected to thermal load, no resinlike products are formed, as is the case with biodiesel.

Diethyl oxalate has adequate volatility and therefore does not accumulate in the engine oil. Moreover, a significant reduction in emissions was established when diethyl oxalate was added to fossil diesel engine fuel. Experiments with a test engine showed that a mineral diesel engine fuel mixture containing 16.2% by volume of diethyl oxalate as a mixture component produced 50% fewer soot emissions than a diesel engine fuel without this component. At the same time, the efficiency of the engine and its potential performance decreased only marginally with no modifications having been made to the fuel injection system. Clearly, the lower combustion heat of the diethyl oxalate is partially compensated by the significantly higher density and the improved efficiency.

This effect is surprising in that diethyl oxalate has a very low-energy density compared with conventional diesel engine fuel and was contained in relatively high proportions as a mixture component. The experiments also showed that a relatively high input of diethyl oxalate has no adverse effects on the ignition quality of the fuel mixture.

Since diethyl oxalate consists exclusively of vegetable products, it is a bio propellant within the terms of the current EU directive. The use of diethyl oxalate as an additional mixture component or additive for diesel engine fuels enables the proportion of bio fuels to be significantly increased without adverse effects. The use of a conventional biodiesel would lead to further increases in the existing overcapacities of glycerol.

The invention is now to be explained by means of an example:

8 kg of a commercially available, pure mineral diesel engine fuel (density 833 kg/m³) with a calorific value of H_(u)=42935 kJ/kg and a cetane number of 55.2 were admixed with 2 kg of diethyl oxalate (density 1079/m³) by simple stirring. The mixture had a calorific value of H_(u)=38150 kJ/kg and a cetane number of 60.1.

A one cylinder stationary engine with a power rating of 75 kW was operated on a test bed using the fuel mixture prepared as above at four different engine speeds up to a maximum engine speed of 1500 min⁻¹ which was near maximum load. The following values were determined in the exhaust gas for the four different operating conditions: visual opacity, gravimetrically determined particulate emissions and emissions of carbon monoxide and hydrocarbons.

The fuel consumption was also determined for each operating condition.

The engine was then run with the mineral fuel without the addition of diethyl oxalate under the same conditions (engine speed and load condition) which were set by modifying the fuel metering (injection).

A comparison of the measured values determined for a fossil diesel engine fuel with and without the addition of diethyl oxalate produced the following results:

The use of a diesel engine fuel mixture containing 16.2% by volume of diethyl oxalate significantly reduced the opacity of exhaust gases by about 50% compared with the additive-free diesel engine fuel. The values for particulate emissions fell by approximately 43%.

The emission values for carbon monoxide fell by 16% to 22% and those of the hydrocarbons by 4% to 29%, dependent on engine speed.

Fuel consumption rose by approximately 10% because of the lower calorific value of the diethyl oxalate. The more complete combustion of the fuel with the addition of diethyl oxalate, which was reflected in the improved emission values, also resulted in improved engine efficiency with a saving of 4 g/kWh. No adverse effects of the addition of diethyl oxalate on the operating performance of the engine were detected during the period of 10 hours during which the engine was run. Diethyl oxalate can therefore be used as a mixture component for diesel engine fuels without any problems arising. 

1-8. (canceled)
 9. A liquid fuel selected from the group consisting of a diesel engine fuel mixture and a light heating oil, the liquid fuel comprising: at least one main component selected from the group consisting of a fossil diesel engine fuel, a biodiesel and vegetable oil; additives; and diethyl ester of oxalic acid (diethyl oxalate) as an additional mixture component.
 10. The liquid fuel according to claim 9, wherein said diesel engine fuel mixture contains as said main component said fossil diesel engine fuel and up to 25% by volume of said diethyl oxalate as said additional mixture component.
 11. The liquid fuel according to claim 9, wherein said diesel engine fuel mixture contains as said main component said biodiesel and up to 50% by volume of said diethyl oxalate as said additional mixture component.
 12. The liquid fuel according to claim 9, wherein said diesel engine fuel mixture contains as said main component said vegetable oil and up to 50% by volume of said diethyl oxalate as said additional mixture component.
 13. The liquid fuel according to claim 9, wherein the light heating oil is a crude oil raffinate that contains up to 25% by volume of said diethyl oxalate as said additional mixture component.
 14. The liquid fuel according to claim 9, wherein the light heating oil contains a mixture of said vegetable oil with up to 50% by volume of said diethyl oxalate.
 15. A method of formulating a diesel engine fuel mixture, which comprises the steps of: mixing diethyl oxalate as a diesel engine fuel in combination with additives for formulating the diesel engine fuel mixture.
 16. A method of formulating a light heating oil, which comprises the steps of: mixing one of a crude oil raffinate and vegetable oils with diethyl oxalate as a mixture component for formulating the light heating oil. 